1. Field of the Invention
This invention relates to a line pressure control system for an automatic transmission of a vehicle, and more specifically to a control system for controlling a line pressure by controlling a pressure regulating valve through a solenoid thereof.
2. Description of the Related Art
Generally, an automatic transmission is provided with a torque converter and a multiple stage transmission gear mechanism with a planetary gear mechanism. The transmission gear mechanism is provided with a plurality of frictional elements, such as brakes and clutches for switching a power transmitting path therein to establish a desirable shift gear staged. The frictional elements are adapted to be controlled by a hydraulic control circuit to be engaged and disengaged to make a shift operation. Since the line pressure seriously affects engaging force and switching speed of the frictional elements, or shift shock in shift operation, it is necessary to deliberately control the line pressure.
There has been known a line pressure control system for electrically controlling the line pressure of a hydraulic control circuit of the transmission mechanism which includes a pressure regulating valve for controlling the line pressure of which pilot pressure is controlled by a duty solenoid valve. According to the conventional line pressure control system, the line pressure is controlled by a duty ratio signal for the duty solenoid valve so that a desirable frictional engaging force is obtained in accordance with the magnitude of engine torque. Alternatively, the line pressure is controlled to obviate the shift shock during the shift operation. In this case, it is necessary to properly control timing of a switching operation and a change of the engaging force of the frictional elements between the engaged and disengaged position. Otherwise, a torque shock is undesirably produced, because of an improper change in the turbine and the engine speeds.
In view of these problems, it has been known to control the line pressure in terms of a time period of the shift operation. For example, Japanese Patent Publication 63-3183 discloses a learning control of the line pressure for controlling the frictional elements wherein the time period of the shift operation is measured for changing the line pressure in a manner that the time period of the shift operation is controlled to a target value. The line pressure control disclosed in the Japanese Patent Publication 63-3183 is intended to make the time period of the shift operation constant to prevent shift shock and restrict undue slip in the frictional elements.
It should, however, be noted that the above learning control of the line pressure is disadvantageous in the following points.
In one condition of the shift operation, a frictional element is associated with a one way clutch to accomplish a shift operation. In the other condition, one frictional element is released or goes off and another frictional element is engaged or comes on to accomplish a shift operation. In the one condition of the shift operation, the shift operation can be made without serious torque shock, since a power transmitting path can be switched smoothly. However, in the other condition of the shift operation, releasing timing of the one frictional element is not determined in accordance with engaging timing of the other frictional element so that a change rate of capacity for transmitting torque through the one frictional element does not correspond to that for the transmitting torque through the other frictional element. As a result, even when the torque capacity of the one frictional element is reduced, turbine speed may produce temporarily an abrupt increase where the torque capacity of the other frictional element is not increased sufficiently. Secondly, when the torque capacity of the other frictional element is increased sufficiently, the turbine speed is reduced quickly to produce a depression of the turbine speed. Thirdly, when the one frictional element keeps a large amount of capacity and the other frictional element increase the torque capacity thereof rapidly, both of the frictional elements are kept in a substantially engaged condition so that the engine torque is abruptly reduced to produce a torque depression as well. Thus, the element will be deteriorated in durability and reliability even where the time period for the shift operation is precisely controlled.